“Sure, you're still human - on the outside! Inside, though, you're seemingly part Ghoul, which doesn't have a downside, except that the perk doesn't function away from radiation. This negates any Rad-X you may need to take (allowing you to sell it instead), and you can venture into the Glowing Sea in just your skivvies if you wish. Simply reference locations in this guide's Map chapter that mention radiation (especially severe) and you have an additional "recharge" point.”— Fallout 4 Vault Dweller's Survival Guide description
This perk functions by healing a multiplier of any rad poisoning received. At rank 1, 50% of rad poison received is healed as health; at rank 2, 100%; at rank 3, 150%; and at rank 4, the radiation level will reduce slowly. This does not provide a constant regeneration effect based on current radiation poisoning (as the similarly-worded Fallout: New Vegas perk Rad Child did).
This has the ramification that this perk has a negative synergy with rad resistance. The more rad resistance one has, the less rad poisoning received, which reduces how much health is restored.
This effectively increases the healing done of all uncooked food items, since the rad poisoning they bestow will also restore additional health.
This perk does not prevent the accumulation of rads, so as one's health regenerates, the red portion of the health bar will also grow. This means that if one doesn't have RadAway or Mysterious serum, radiation will still eventually prove lethal.
With Rank 4, radiation is reduced only during active game time, not while sleeping or waiting. As of patch 1.7.20, it does not drop during fast travel.
Rank 4 has a rad reduction rate of 1 rad/sec. This is 4 times slower than the rad reduction offered by Rank 2 of the Solar Powered perk. However, unlike Solar Powered, this effect is always active.